$(document).ready(function() {
    // Cache selectors for faster performance.
    var $window = $(window),
  	$mainMenuBar = $('#mainMenuBar'),
  	$mainMenuBarAnchor = $('#mainMenuBarAnchor');
       
          // Run this on scroll events.
        //scroll()
       //当用户滚动指定的元素时，会发生scroll事件。
       //scroll事件适用于所有可滚动的元素和window对象（浏览器窗口）
       //scroll()方法触发scroll事件，或规定当发生scroll事件时运行的函数
  	$window.scroll(function() {
      //scrollTop()方法返回或设置匹配元素的滚动条的垂直位置
      var window_top = $window.scrollTop();
    //javascript用offsetTop();jquery用offset().top;
      var div_top = $mainMenuBarAnchor.offset().top;
      if (window_top > div_top) {
          // Make the div sticky.
          $mainMenuBar.addClass('stick');
          $mainMenuBarAnchor.height($mainMenuBar.height());
      }
      else {
          // Unstick the div.
          $mainMenuBar.removeClass('stick');
          $mainMenuBarAnchor.height(0);
      }
		});
});
